This invention relates to automatic speed change control apparatus, more particularly to the change of shift modes of electro-fluid pressure type automatic speed change apparatus of a vehicle, especially a civil work car such as a motor scraper or an off-the-road dump truck.
In a motor scraper or an off-the-road dump truck, is used a multi-stage gear transmission system driven by a torque converter provided with a directly coupled clutch, whereas in a bulldozer or the like a direct drive type multi-stage gear transmission system is not provided with a torque converter (this type of transmission system provides a low cost speed changer when it is combined with a gas turbine). The electro-fluid pressure type automatic speed change control apparatus of this invention is applicable to both types of the gear transmission systems described above, and the application thereof to such civil work cars as a motor scraper and bulldozer is extremely advantageous.
Electro-fluid pressure type automatic speed change control apparatus are disclosed in U.S. Pat. Nos. 3,448,640 and 3,572,168 of the titled "Control Apparatus Utilizing Semiconductors" but if such control apparatus are applied to civil work cars, there are the following problems.
More particularly, since in a civil work car the load under operating condition varies rapidly and since the running resistance of the road in the operating field varies greatly, in most cases a multi-stage gear transmission system having six, eight or ten stages is used for the speed changer for transmitting a high torque. However, if an eight stage automatic speed changer designed for a larger load is used on a road manifesting a small running resistance, when the car is operated for the same distance, the cycle time becomes longer thus decreasing the amount of soil carried when compared with a six stage automatic speed changer having the same maximum speed. To obviate this difficulty it has been proposed an eight stage automatic speed changer in which the first stage is manually selected and from the second to the eighth stage the speed is automatically changed. Hereinafter this type of speed changer is termed "eight(8) stage, second to eighth stage (2-8) automatic speed changer". With this construction, when the car is running under a high load or a slope it is necessary to manually select the first stage, which is of course troublesome and inconvinient. This is also true in a six stage automatic speed changer in which the first stage is manually selected and from the second to the sixth stage the speed is changed automatically.
As above described where the automatic speed change is made possible between the first to sixth stages the range of the speed change would be fixed from the first to sixth stage whereas when the automatic speed change is made possible between the second and sixth stage the range of the automatic speed change would be fixed to from the second to sixth stage. Thus, the shift mode of automatic speed change (the range of the speed stage is which automatic speed change is possible) is limited to only one and is not variable. In other words, it is impossible to vary the lower limit of a predetermined range in which automatic speed change is possible (for example, a range from the forward second stage to the forward sixth stage). Thus, for example, when the lower limit of the automatic speed change is the forward second speed, it has been impossible to include the forward first speed in the automatic speed change range. For this reason, when the car is running under a high load or along a slope, it is necessary to perform a troublesome range changing operation such that the control lever is manually returned to the first stage and then set the control lever to the automatic speed change range after the car has accelerated. Furthermore, in the prior art automatic speed change control apparatus disclosed in said U.S. Patents, when the throttle opening is small, the speed changing point is set on the low speed side but when the throttle opening is large the speed changing point is set on the high speed side so as to select an optimum speed stage is accordance with the load of the engine.
For this reason, the shift up speed at the minimum throttle opening is on the low speed side. For example, when the car is running down a steep slope by decreasing the throttle opening and by selecting a low speed stage for applying an engine brake, as the car descends the slope the speed increases so that it is impossible to efficiently utilize the engine brake since the speed stage is automatically shifted up in response to such increase in the engine speed.
For this reason, it is necessary to manually operate the speed change range selecting lever to hold it at a low speed stage each time it becomes necessary to apply the engine brake.
Since civil work cars are subjected to a rapid change in the load or running resistance of the road, in most cases multi-stage gear transmission systems having 6.8 or 10 stages are used for the purpose of transmitting high torques. In an automatic speed changer having such multi-stage gear transmission system, the number of automatic speed changes is also large. However, when the number of the range selection stages for selecting the automatic speed change range is made equal to the number of speed change stages the operator can not immediately judge that which one of the range select stages should be selected. To make easy judgement of the operator, it is advantageous to construct the range selecting lever to use the same pattern of selecting the range as a general passenger car inspite of a large number of speed change stages.
The select lever pattern of a conventional speed change generally follows a shift order of P(parking) - R(reverse) - N(neutral) - D(drive range). Although it is necessary to set the range select stage is accordance with the maximum car speed, the maximum speed of the civil work car is at most 60 km/h so that it is not necessary to provide all speed ranges for each position of the select lever. For this reason, even when the number of the speed change stages is large, the shift pattern may be, for example P-R-N-D-2-1, and the automatic speed change ranges may be D-2-1, wherein 2 corresponds to the second range selection stage (position) and 1 the first range selection stage (position). In such a case, there is a problem that which one of the speed stages of the multi-stage speed changer should be assigned to a limited number of the range selecting stages. Although the maximum speed stage of the multi-stage speed changer is assigned to the driving range D, an intermediate speed selection stage should be determined by considering the relationship between the maximum car speed and the speed reduction ratios between respective speed stages. Accordingly, even if the number of the speed change stages is the same, when the specification (maximum car speed, reduction gear ratio, etc.) of the multi-stage speed changer is changed it is necessary to change the speed stages corresponding to respective range selection stages. However, this requires a large change of the specification of the automatic speed change control apparatus.
It is desirable that the range selecting lever pattern (P-R-N-D-2-1) or the number of the range selection stages described above should be the same regardless of the variation in the number of the speed change stages of the multi-stage automatic speed changer. For example, when the range selecting lever pattern is changed whenever the type of the car is changed, the burden of the operator becomes heavy. Accordingly, the range selection lever pattern should not be changed. For example, let us consider a case in which the mode of the range selecting stage is made to be the same as the above described mode P-R-N-D-2-1 for a forward six stage automatic speed changer and a forward eight stage automatic speed changer. Further, let us assume that the forward six stage automatic speed changer and forward eight stage automatic speed changer are set to the same maximum speed for the drive range D and that the same car speed is set for the second stage of the second-range and the first stage of the first range. Accordingly, in view of the speed reduction ratio of the speed changer, the range select stages of the forward six stage speed changer and the forward eight stage speed changer are different from the actual speed stage. For example, where the drive range select stage D of the forward six stage automatic speed changer is at the forward sixth speed, the second range select stage 2 is at the forward fourth speed and the first range select stage 1 is at the forward first speed, the drive range D of the forward eight stage automatic speed changer will be at the forward eight speed, the second range select stage at the forward fifth stage and the first range select stage at the forward first speed.
In this manner, in a multi-stage speed changer, as the number of the speed change stages varies, the speed stage corresponding to the range select stage becomes different. Accordingly, in an automatic speed change controll apparatus, it is necessary to control such that a predetermined speed stage corresponding to the predetermined select stage to be selected will be comensurate with the number of the speed change stages of the multi-stage speed changer to be controlled. For this reason, the construction of the automatic speed change control apparatus will be different when the number of the speed change stages and the maximum speed of the multi-stage speed changer to be controlled vary. However, it causes increase in the manufacturing cost and prevents mass production to make different the construction of the automatic speed change control apparatus for a slight difference in the number of the speed change stages.
In the automatic speed change of a bulldozer or a like civil work car automatic speed change is important during reverse running just like in the forward running so that it is important to set a speed change range (the range of the speed change stages) in response to the load condition during the forward and reverse runnings. For example, it is necessary to set upper and lower limits of the speed range in which automatic speed change is effected so as to select a speed stage suitable for a particular operating condition of the car in accordance with forward soil transportation (running), forward soil pushing (bulldozing), forward heavy digging (ripping), reverse soil preparation (running), reverse running, etc.